ci: pre-populate synthtool cache to avoid race condition (#17230)

This PR resolves a blocking parallel generation issue on GitHub Actions
and updates Spanner's post-processing config to align with recent
template updates.

### 1. Fix GHA parallel generation race condition
When running `librarian generate --all` in GHA, multiple libraries are
generated in parallel. They concurrently invoke `synthtool`, which races
to clone/update the templates repository (`googleapis/synthtool`) in the
shared cache directory `~/.cache/synthtool/synthtool`.

This results in either:
* A **clone race**: One process starts cloning, and others see the
half-created directory, assume it's valid, and fail with `fatal: failed
to resolve HEAD as a valid ref` when running `git branch`.
* An **update race**: Multiple processes try to run `git checkout` /
`git pull` concurrently, failing with `fatal: Unable to create ...
index.lock: File exists`.

**Fix**: 
We now clone `googleapis/synthtool` once sequentially at the beginning
of the workflow (`Clone Synthtool Templates`), and configure
`SYNTHTOOL_TEMPLATES` environment variable globally to point to this
local directory. This completely bypasses all concurrent git clone/pull
operations during parallel generation, resolving the race conditions and
speeding up the build.

### 2. Fix Spanner post-processing AssertionError
We removed the obsolete `Supported Python Versions` replacement block
from
`.librarian/generator-input/client-post-processing/spanner-integration.yaml`.

**Why it was needed**:
* On **May 19, 2026** (commit `2408166fec`), Python 3.7-3.9 support was
dropped specifically for Spanner. Because the `synthtool` templates
still generated `Python >= 3.9` in `README.rst` by default, a
post-processing workaround was added to force it to `3.10`.
* On **May 21, 2026** (commit `e643ce8` in `googleapis/synthtool`),
`synthtool` templates were updated globally to drop Python 3.9 support
for all mono-repo libraries.
* As a result, the freshly generated `README.rst` for Spanner now
already defaults to `Python >= 3.10`. The May 19 workaround became
obsolete and failed with `AssertionError: Replaced 0 rather than 1
instances` because the "before" pattern (expecting `3.9`) no longer
exists.

TAG=agy
CONV=acb0ee9b-3d87-4b9e-b3c6-0d4aa3efbd35

Author: suztomo

.github/workflows/regenerate-all.yml

@@ -14,6 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     env:
       PANDOC_VERSION: 3.8.2
+      SYNTHTOOL_TEMPLATES: /home/runner/synthtool/synthtool/gcp/templates
 
     steps:
       - name: Checkout
@@ -41,6 +42,10 @@ jobs:
           version=$(sed -n 's/^version: *//p' librarian.yaml)
           go run "github.com/googleapis/librarian/cmd/librarian@${version}" install
 
+      - name: Clone Synthtool Templates
+        run: |
+          git clone --recurse-submodules --single-branch https://github.com/googleapis/synthtool.git /home/runner/synthtool
+
       - name: Regenerate
         run: |
           version=$(sed -n 's/^version: *//p' librarian.yaml)

.librarian/generator-input/client-post-processing/spanner-integration.yaml

@@ -607,38 +607,7 @@ replacements:
       Next Steps
     count: 1
 
-  - paths: [
-      packages/google-cloud-spanner/README.rst
-    ]
-    before: |
-      Supported Python Versions
-      \^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^
-      Our client libraries are compatible with all current `active`_ and `maintenance`_ versions of
-      Python\.
-
-      Python >= 3\.9, including 3\.14
-
-      \.\. _active: https://devguide\.python\.org/devcycle/#in-development-main-branch
-      \.\. _maintenance: https://devguide\.python\.org/devcycle/#maintenance-branches
 
-      Unsupported Python Versions
-      \^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^\^
-      Python <= 3\.8
-    after: |
-      Supported Python Versions
-      ^^^^^^^^^^^^^^^^^^^^^^^^^
-      Our client libraries are compatible with all current `active`_ and `maintenance`_ versions of
-      Python.
-
-      Python >= 3.10, including 3.14
-
-      .. _active: https://devguide.python.org/devcycle/#in-development-main-branch
-      .. _maintenance: https://devguide.python.org/devcycle/#maintenance-branches
-
-      Unsupported Python Versions
-      ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-      Python <= 3.9
-    count: 1
 
     # Note: noxfile.py is heavily customized so we clobber the whole file. 
   - paths: [